The present invention relates generally to gas turbine engines. More specifically, this invention relates to a system for improved attenuation of the noise generated by the interaction of rotor blades and stationary guide vanes in gas turbine engines.
Noise has been a significant negative factor associated with the commercial airline industry since the introduction of the aircraft gas turbine engine. Considerable effort has been directed toward quieting aircraft engines. The aerodynamic interaction of rotor blades and stationary guide vane wakes adds a significant contribution to the noise produced by the jet engine. Fans and compressors include at least one row of a plurality of circumferentially spaced apart rotor blades for compressing air channeled therethrough, and a row of circumferentially spaced apart stator vanes axially spaced apart from the rotor blades. The rotor blades rotate about a longitudinal centerline axis of the engine at a rotational speed and effect a tonal noise at a blade passing frequency (BPF). Interaction tones are generated in the region between the rotor blades and the stator vanes, within the annular duct surrounding the blades and the vanes, conventionally known as spinning modes.
The blade rotation-wakes of the air compressed by the fan blades form rotational pressure fields and impinge on the stator vanes, thereby creating the spinning mode tones. The spinning mode tones occur at discrete frequencies including the fundamental blade passing frequency BPF, alternatively referred to herein as the first harmonic, and higher order frequencies including the second, third and higher harmonics. When this spinning mode rotational speed is fast enough to cause the local Mach number to be greater than about 1.1, the spinning mode tones will propagate outside, either upstream through the duct inlet or downstream through the duct outlet, or both, enhancing BPF tone levels generated directly by the rotor blades.
In order to reduce the spinning mode noises, at takeoff or landing approach for example, it is known in the art to direct the spinning mode tones to impinge on the walls of the interior of the engine, which is lined with a sound absorbent material. This technique causes the spinning mode tones to decay before exiting the engine, examples of which are described in U.S. Pat. No. 3,937,590 issued to Mani on Feb. 10, 1976 and U.S. Pat. No. 4,104,002 issued to Ehrich on Aug. 1, 1978. However, acoustical wall treatment has only made small reductions in fan inlet noise levels and this is compounded by inlet length-to-radius ratios becoming smaller.
In U.S. Pat. No. 4,300,656 issued to Burcham on Nov. 17, 1981, Burcham describes an acoustic noise elimination assembly having the capability to disrupt the continuity of rotating fields of sound pressures forwardly projected from fans or rotors of a type commonly found in the front or compressor first stage of air-breathing engines, when operating at tip speeds in the supersonic range. The assembly includes a tubular cowl defining a duct for delivering an air stream axially into the intake of a jet engine and a sound barrier, defined by a plurality of intersecting flat plates or struts having a line of intersection coincident with a longitudinal axis of the tubular cowl which serves to disrupt the continuity of rotating fields of multiple tonal components of noise.
Reduction of the spinning mode noise can be achieved by reduction of the production processes at the source of the noise which reduces the incident aerodynamic unsteadiness or the mode generation from such interactions. It is conventionally known to select the number of vanes and the number of blades to create a spinning mode propagation cut-off phenomenon. For example, Gliebe et al. describe a low noise fan assembly in their U.S. Pat. No. 5,169,288 issued on Dec. 8, 1992, and suggest that the number of vanes is selected for a predetermined number of blades for obtaining substantially equal values of a cut-off ratio for at least two blade passing frequency harmonics for reducing spinning mode noise from the fan assembly. In practice, for the spinning mode propagation cut-off a number V of vanes and a number B of blades are selected to achieve Vxe2x89xa72.3B. In some designs, however, particularly in high by-pass turbofan engines requiring a relatively large number of rotor blades, a cut-on fan-stator (V less than 2B) may be selected in order to find a compromise with other design criteria. In such cases the spinning mode is always cut-on, resulting in increasing the BPF tone level. Therefore, there is a need for an alternative solution to prevent spinning mode generation.
It is one object of the present invention to provide a method of reducing rotor-stator interaction tone noise in fans and compressors.
Another object of the present invention is to provide a method of inhibiting spinning mode tone propagation to reduce tone noise levels of fans or compressors.
Yet another object of the present invention is to provide a method of inhibiting generation of a spinning mode in fans and compressors to reduce tone noise levels thereof.
A further object of the present invention is to provide an improved fan or compressor assembly effective for reducing BPF tone and harmonics.
In accordance with one aspect of the present invention, a method is provided for reducing rotor-stator interaction tones in an annular duct surrounding a plurality of circumferentially spaced rotor blades and a plurality of circumferentially spaced stator vanes axially spaced apart from the rotor blades, the annular duct including an inlet for receiving air and an outlet for discharging at least a portion of the air compressed by the rotor blades. The method comprises a step of providing a controlled air blowing system for blowing an air flow in a direction opposite to the rotation of blade-wakes into the annular duct between the rotor blades and the stator vanes to impart an opposite rotational momentum with respect to the air compressed by the rotor blades, thereby reducing the strength of a spinning mode generated by a symmetrical pattern of the blade rotation-wake of the compressed air.
It is preferable to blow in a hot air flow to increase the temperature of the compressed air, thereby increasing a local sound speed when sound waves propagate in the air medium. It is also preferable to blow the air flow asymmetrically into the annular duct to destroy the symmetrical blade rotation-wake pattern, thereby preventing the generation of the spinning mode.
In accordance with another aspect of the present invention, a gas turbine engine fan assembly comprises a plurality of circumferentially spaced rotor blades and a plurality of circumferentially spaced stator vanes axially spaced apart from the rotor blades. An annular duct surrounds the rotor blades and stator vanes, and has an inlet for receiving air and an outlet for discharging at least a portion of the air upon compression by the blades. Means are provided for blowing an air flow in a direction opposite to the rotation of the rotor-blade wakes into the annular duct between the rotor blades and the stator vanes to impart an opposite rotational momentum with respect to the air compressed by the rotor blades, thereby reducing the strength of a spinning mode generated by a symmetrical pattern of the blade rotation-wakes of the compressed air.
The means preferably include a plurality of nozzles in an outer wall of the annular duct and the nozzles are connected to a compressed air source of the gas turbine engine for controllably blowing the air flow. It is preferable that the nozzles are circumferentially spaced apart in an asymmetrical pattern to blow the air flow asymmetrically into the annular duct to destroy the symmetrical blade rotation-wake pattern, thereby preventing the generation of the spinning mode. It is also preferable that each of the nozzles is oriented in a transverse cross-section, angularly with respect to a diametrical line of the annular duct passing the nozzle so that the air flow is adapted to controllably penetrate a radial distance ranging from 5% to 20% of a length of the fan radius when the means are in operation.
The present invention provides a solution effective for source reduction of rotor-stator interaction tone noises of a gas turbine engine fan assembly to significantly reduce the fan tone level. Other advantages and features of the invention will be better understood with reference to a preferred embodiment described below.